The invention relates to a circuit for controlling currents flowing through windings of stepping motors of the type in which both ends of all the windings are individually accessible for the purpose of electrical connections. The term "winding" as used herein means winding or winding section.
As is known, stepping motors are used as shaft drives for machine tools, peripheral equipment for computers, clocks or the like. Such stepping motors require a circuit which on the one hand permits the current to rise rapidly to the necessary value in a prescribed sequence in individual motor windings and on the other hand permits the current to drop again rapidly to zero after a given time which is dependent on the stepping frequency.
There are already known various circuits for control of this kind for stepping motors. Such known circuits are described, for example, in the technical reference book The Stepping Motor Handbook, published by Sigma Instruments, Inc., Braintree, Mass., U.S.A. cop. (1972), German Ed., cop. (1973), hereinafter referred to as the "Handbook".
(A) An R-L-circuit is one of the circuits described in the Handbook. Here an ohmic resistance is connected in series with the motor winding and a high voltage is applied to this combination. This resistance has two different effects. It limits the current to the necessary value and in accordance with the law .tau.=L/R, ensures a small motor constant, which leads to a rapid rise in the current. In order to obtain a rapid current disconnection, a resistor or Zener diode is connected in series with a normal diode in the free running circuit. The circuit can be used both for stepping motors permitting a separate access to all the winding ends and for stepping motors which give access to only half of all the winding ends and permit a common neutral point or partial neutral points. The principle is shown in FIG. (a), representation 36 on p. 47 of the Handbook. PA0 (B) A constant current chopper control circuit is described in FIG. (b), representation 36 on page 47 of the Handbook. This circuit fulfils the requirement of rapid switching on, maintaining of constant current, rapid switching off, and relatively small losses. However, it requires a high voltage source and a relatively complicated current control circuit for each motor winding. PA0 (C) A rapid excitation circuit (superimposition of an auxiliary voltage) is described in FIG. (c), representation 36 on page 47 of the Handbook. This circuit fulfils the requirements made in (B) above, but requires one current measuring circuit per winding and two voltage sources, which one has a low voltage and the other a high voltage. PA0 (D) A circuit described in FIG. (d), representation 36 on page 47 of the Handbook, relates to a voltage control intended for stepping motors. The advantages and disadvantages of the control are described in detail on pages 56 to 58 of the Handbook. U.S. Pat. No. 3,560,821 also describes a stepping motor control of this type. One disadvantage of this circuit is that due to the relatively long response time, at the most from 3.3 ms (milliseconds) to 10 ms, of the voltage regulating circuit caused by the multiple line frequency of from 100 Hz (Herz) to 300 Hz, there are system-dependent power restrictions with regard to the starting and stopping behaviour of the stepping motor. Another disadvantage is that the stepping frequency change speed on passing from high to low stepping frequencies must also be limited, because otherwise there would be excessive switching currents in the just-activated windings leading to magnetic saturation and high losses in the switching transistors through exceeding of the saturation voltage between collector and emitter. These effects occur because the smoothing capacitor located at the voltage regulating output must discharge from a high to a low voltage on passing from high to low stepping frequencies.